Automobile power steering devices are in wide use. The majority of the power steering devices are hydraulic, whereby a hydraulic pump providing a steering force is constantly driven by the power of the engine. The constant driving of the hydraulic pump, applied irrespective of the need for steering, contributes to poor fuel economy. On the other hand, electric power steering uses an electric motor to provide a steering force, and drives the electric motor only when steering the vehicle. The fuel saving effect is thus much greater than that of the hydraulic power steering device. Because the steering force to produce in comparison with the hydraulic power steering is small as for the electric power steering, the use of electric power steering device has limited to relatively smaller and lighter vehicles. However, the number of vehicles with the electric power steering system has been increasing because of the better fuel efficiency.
The sliding area of an electric power steering is configured from a steel worm shaft and a worm wheel having resin teeth. The resin typically uses nylon, a type of polyamide. Some of the properties required for a grease as a lubricant that is used in such a sliding area include a low friction property for improved transmission efficiency, and an anti-stick-slip property for stably maintaining a low torque over extended time periods.
A number of greases mixed with various waxes have been proposed as lubricants for use in electric power steering devices having such a steel-resin sliding area. Examples include a grease as a mixture of a thickener and a base oil with a montan wax (PTL 1), a grease as a mixture of a thickener and a base oil with a polyethylene oxide-based wax (PTL 2), and a grease as a mixture of a thickener and a base oil with a carboxylic acid amide-based wax (PTL 3). Another example is a solid lubricant, or a grease of primarily polytetrafluoroethylene (PTL 4). Yet another example is a grease mixed with an ionic liquid (PTL 5).
These greases all have low coefficients of friction or a low torque. A problem, however, is that the grease tends to gradually become removed from the sliding area under the friction over a long period of use, and then the oil film finally breaks down and possibly causes stick-slip. Stick-slip may cause resistance in the movement of the steering wheel, and may spoil the steering feel in situations where high output is needed such as in parking a car.